A number of electronic applications utilize resistors and analog integrated circuits that include such resistors are well known. Resistors are optimally linear devices, that is, the output is directly proportional to the input. However, achieving a highly linear resistor in an integrated circuit is difficult or expensive. There are two principal types of resistors available using integrated circuit technology but both have linearity problems.
A diffused resistor employs an N-type or P-body diffusion as a resistive layer or body. However, it is quite expensive to produce an integrated circuit with a diffused resistor body that is fully isolated by silicon oxide. Typically, electrical isolation of the resistor body from the surrounding silicon is obtained by reverse biasing the PN junction or parasitic diode where the diffusion body meets the silicon substrate. A problem with this reverse biasing approach is that the depletion region of the PN junction reduces the effective size of the diffusion resistor body. Further, the resistive value of the resistor (the effective thickness of the diffusion resistor body) varies as a function of different polarization conditions. That is, since the region depleted from freecharge does not contribute to conduction, the resistive value of a given resistor will vary with polarization. Thus, this type of resistor exhibits nonlinear behavior.
FIG. 1 shows a resistor portion 10 of an integrated circuit with isolated resistor modules 21 and 22. Each of the resistor modules 21 and 22 includes a more heavily doped contact region 23, 25, respectively, and terminals 41, 43 making direct electrical contact with the heavily doped contact regions 23, 25, respectively. Also, FIG. 1 shows the depletion region 29 of each of the resistor modules 21 and 22 formed at the PN junction where the resistor module is surrounded by the substrate 30. It will be understood that while FIG. 1 shows the resistor modules 21, 22 formed of N-type material, and the substrate 30 formed of P-type material, the resistor body may be P-type while the substrate may be N-type.
Yasushi, Japanese Patent Publication No. 05-175429, published Jul. 13, 1993, discloses an integrated circuit resistor element in which a resistor 20 is formed by P-type diffusion in an N-diffused island 21 surrounded with an isolating area 11, as illustrated in FIG. 7. A power supply 26 is connected to electrodes 22 and 23 of the resistor 20, such that the voltage on the electrodes 22 and 23 is also received by voltage followers 27 and 28 and such that DC power supplies 29 and 30 also provide current to electrodes 24 and 25 of the N-diffused island 21. Yasushi discloses that the DC power supply 29 and the DC power supply 30 are additionally connected to electrodes 24 and 25, respectively, of the island 21. Accordingly, Yasushi discloses that, when the voltage of electrode 22 of resistor 20 equals Vra and the voltage of the DC power supply 29 is Vs, the potential of electrode 24 will equal Vra+Vs. Similarly, when the potential of electrode 23 equals Vrb, then the potential of electrode 25 will equal Vrb+Vs, and accordingly, the voltage drop across resistor 20 and the voltage drop across the N-diffused island 21 become equal, such that a constant reverse bias is applied in this state.
Thus, Yasushi discloses extra power supplies provided to each of electrodes 24 and 25 of N-diffused island 21 surrounding resistor 20 to maintain an equal voltage between the portions of N-diffused island 21 and resistor 20. Also, although the voltage drop disclosed by Yasushi is equal between terminals 22 and 23 of resistor 20 and terminals 24 and 25 of N-diffused island 21, the voltage Vra at electrode 22 of resistor 20 will not equal the voltage Vra+Vs at electrode 24 connected to N-diffused island 21, and similarly, the voltage Vrb at electrode 23 of resistor 20 will not equal the voltage Vrb+Vs at terminal 25 of N-diffused island 21.
Yasushi explains that DC power supplies 29 and 30 are power sources for supplying an electrical potential difference higher than the output voltage.
A polysilicon resistor also shows linearity problems. When a mean voltage reaches a certain value, the resistor will start to exhibit nonlinear behavior. In this case, the ambient temperature has an effect on the resistance. In many technologies, only low resistive polysilicon is available, which is useful for integrated circuit applications with resistors that can have a range of a tenth of an Ohm to a few KOhms. First, when the mean voltage drop on the monocristalline grain reaches a certain value, the resistor starts to exhibit significant nonlinear behavior. This value Vt is provided in accordance with the following equation:
      V    t    =            K      ·      T        q  
where K is the Boltsmann constant,
T is the absolute temperature, and
q is the charge,
so, for example, Vt=25 mV at room temperature.
Accordingly, for example, doubling both W and L of the resistor body will not change the resistive value; however, the mean voltage across the single polysilicon grain would be divided by two and so, in this case, the linearity improves. Thus, the linearity of the resistor can be improved, but only at the expense of increasing the size of the resistor body on the integrated circuit.
A high ohmic polysilicon is isolated from the substrate by a thick field oxide, but it has a very low doping level. Also, a weak capacitive coupling occurs between the polysilicon and the substrate, thus causing further nonlinear behavior, since the capacitive coupling can deplete the resistor body or accumulate charge, thus changing the value of the resistance and introducing another variable.
It will be understood that metal resistors, although typically linear, have very low ohmic values (in the mOhm range) for integrated circuit applications.
Several approaches have been used to mitigate the foregoing problems. First, a more heavily doped diffusion may be used for the resistor body. Thus, the depleted region narrows and the same voltage may be applied. Accordingly, linearity improves, however the silicon area required is increased because more heavy doping lowers the current per square unit of area and so, the length of the resistor needs be increased to achieve the same resistive value.
The resistor may be divided into N modules, each module implemented as an isolated well. Such an approach may use N isolated wells, such that each well is biased by shorting a contact region of the well with the correct terminal (typically the higher voltage terminal) of the module.
FIG. 2a shows a resistor body 20 with the high voltage terminal 41 making contact with the more heavily doped contact region 23 of the resistor body 20 such that the high voltage terminal 41 is shorted to the contact 33 of the surrounding N-type conductivity epi layer 30. FIG. 2a also shows the insulation SiO2 layer 50 and the second contact region 25 of the resistor body 20 to which the lower voltage terminal 43 is connected, and also shows the depleted region 29. The example shown in FIG. 2a shows that the N-type epi layer 30 surrounding the resistor body 20 is directly on top of a P-type substrate 39.
FIG. 2b shows a resistor body similar to that shown in FIG. 2a except that the resistor shown in FIG. 2b is comprised of several resistor modules 21 and 22 such that the low voltage contact region 25 of first resistor module 21 is connected to terminal 43 to which the more heavily doped contact region 23b of second resistor body 22 is connected and to which the N-type epi layer contact region 33b of the second module is also connected. Accordingly, resistor modules 21 and 22 are connected in series to form a single resistor 20.
Using such an approach, the mean voltage drop between the resistor body and the surrounding well is divided by the number of resistor modules, and thus, the resistor as a whole behaves more linearly. However, a much bigger silicon area is required to accommodate the resistor modules.
Using a polysilicon resistor, linearity may be improved by increasing both width and length, however, this obviously involves more area and more cost.
In addition, FIG. 5 shows that metal shield 61 of the polysilicon resistor may produce a weak capacitive coupling region 65 between metal shield 61 and resistor body 20, which tends to create an electrical variation that undercuts the linearity of the resistor.
FIG. 6a shows a metal shield connected to a resistor terminal above the body of the diffused resistor to isolate it from charge trapped in the passivation layer or in other portions of the package such that the shield has the same voltage as one terminal of the resistor body and is isolated by an insulation layer, such as an oxide from the resistor body. Such an arrangement can introduce non-linearity near the second terminal of the resistor body with the same effect as the polysilicon resistor shown in FIG. 5.
Accordingly, there is an unmet need for an integrated circuit with a resistor that avoids these and other problems.